Charging device



- L. E. WALKUP CHARGING DEVICE March; 24, 1959 2 Sheets-Sheet 1 FiledJune a. 1955' @500 VAC. PEAK ill/ INVENTOR.

LEWIS E. WALKUP ATTORNEY March 24, 1959 1.. E. WALKUP CHARGING DEVICE 2Sheets-Sheet 2 Filed June 8, 1955 oov CON

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INVENTOR. LEWIS E. WALKUP FIGS ATTORNEY CHARGING DEVICE Lewis E. Walkup,Columbus, Ohio, assignor, by mesne assignments, to Haloid Xerox Inc-., acorporation of New York Appli'cationJune 8,1955, Serial No. 513,934

Claims. (Cl. 250-495) This invention relates to apparatus forelectrically charging an insulating layer to a potential below themaximum electrostatic potential that the insulating layer will holdwithout electrical breakdown, and has for a particular purpose thereofto afford apractical and eificient device for applying an electrostaticcharge of predetermined potential to a photoconductive insulating layer,such as the photoconductive coating, of an electrophotographic plate.

In producing copies for electrophotography, it has been proposed tocharge electrophotographic plates by using a corona charging device suchas is disclosed in. the co pending application of I. J. Rheinfranlc,Serial Number 55,526, filed. October 20, 1948, entitled, Method and.Apparatus for Electrostatically Charging ImageLayers, andInow abandoned.Such a device has been used successfully to. charge electrophotographicplates. How ever, the. device is difiicult to control in, such a manneras to. place a predetermined potential. on the photoconductive'vinsulating layer. An accurately timed drive mechanism is required topass the plate under the charging device at a controlled. rate of speedin order that the amount of charge deposited on any area of the platecoating does not exceed the electrical breakdown strength of, thecoating. Even with a controlled rate of travel of the. plate under thecharging; device, potential variations. are not overcome sincerelatively small voltage variations in the power supply can producerelatively large changes in corona current with correspondingly largevarations in charging rate. Variations in atmospheric pressure alsoaffect the corona current. and charging rate. Differentelectrophotographic plates will. also have; difierent capacities due todifierentthicknesses of insulating layer or of dielectric constant, and,consequently, will require difierent quantities. of, charge to bringthem to the same potential.

It electrophotographic plates are notcharged to a sufficient potential,the electrostatic latent image obtained uponexposure will be relativelyweak and the resulting deposition of electrostatically atttractablematerial, such asa powder or a liquid mist, during development of theimage will be small. If the plates are overcharged, other diflicultiesare encountered. Oneserious resultof. overcharging is the permanentdamage tov the photoconductive layer by the creation. of small spotsorareas. on the plate which are so altered that they are. not thereaftercapable of. holding charge evenv after. recharging the plate.

It has been further found that difierent areas on the same plate oftenbecome charged to-differentpotentials This latter irregularity ofcharge'is. dueperhapsto variations; inthe. photoconductive layer orperhaps in irregus larities in the ion spraying. capacity of different.portions of. the corona discharge device, and theresult ofthisirregularity is astreaky effect when the photoconductive. layer is.dusted with an, electroscopicpowder to producev an image.

It. is disclosed. in myco-pendmg,apphcatromfienal No.

States Patent Patented Mar. 24, 1959 entitled, Charging Device, of whichthis application: is a continuation-in-part, that the introduction of anelectrode such as a conductive grille between the source of ions, e.g.the. corona discharge electrode, and the insulating layer, such as thephotoconductive insulating layer, to which the ions are flowing permitsthe flow of ions to be regulated and' the insulating layer can: be

charged to a potential below the maximum electrostatic:

potential that the insulating layer willv hold., As there pointed out,using this invention. the potential assumed by the insulating layer isalso made relatively independentof. the chargingpotential, the timeofcharging, and other:

characteristics of the charging unit.

If, in the apparatus of my application Serial1No.. 154,295, now US.Patent 2,777,957, the voltage. im pressed on the corona dischargeelectrode. is. an AC.

voltage sufiicient to cause corona discahrge, and a DC. voltageimpressed on the shield or conductive grille which regulates the flow ofions from the corona discharge electrode, highly unusual and beneficialresults are obtained.

Where DC. voltage or pulsed DC. voltage is used for" both the shield andthe corona discharge electrode, the. polarity of the charge placed onthe insulating layer is. determined by the polarityo-f the charge on thecorona Thus, to reverse the polarity placeddischarge electrode. onv theinsulating layer. requires switching the polarity placed. on, the coronadischarge electrode. which is ofa: magnitude between 4,000 to 10,000volts.

ofovercharging is reduced but not eliminated. In contrast to this, WhereA.C. is usedf'for the corona discharge" In addition to determining thepolarity of the charge: placed on the insulating layer, the shield alsodetermines:

the maximum potential of that charge so that overcharging-is not merelyimprobable, but impossible. The charg ing device of the instant.invention also permits charging: even at low atmospheric, pressureswithout damage to.

the plates. Further, the instant invention eliminates the. necessity ofplacing the shield between the coronadischarge electrode and theinsulating layer. shield may consist of conductive wires. spaced betweenthe corona discharge electrode and the insulating plate as set forth inthe said parent case, or, alternatively, may. consist of a rectangularor cylindrical trough with condnctive. back and sides but withnoconductive member between the corona discharge electrode and theinsulating.

layer to be charged, or. finally, the shield may consist simply of aflat conductive member positioned in. back of the coronadischarge'electrode, ie on the side away from the insulating layer to becharged and extendedsufiiciently far laterally beyond the coronadischarge electrode to prevent corona discharge-past the edge of theshield;

For a better understanding of this invention, reference is now directedto the following description taken in.con-

nection with the accompanying drawings, and the scope: of my inventionwill be pointed out in the. appended claims. In the drawings, Fig. l isaperspective view of? a cornoa. charging device having a controlelectrodein. accordance with a preferred form of my invention; and

In addition,., the DC potential placed. onthe shield does not abso--lutely prevent overcharging of the insulating layer but; ratherdetermines a. potential. above which therate of charging issubstantially reduced sothat. the possibility Thus, the.

. 1 S, v I a 3 7 1 5 1 we out rrrr- 5O bstantially paraklel toi the: cordischarge wires 5. The belts run on pulleys 19 aflixed to shafts 13 and14, the shafts and pulleys being placed with their axes below the planeof guide plate 3 so that the belts and 11 slide along the top face ofplate 3 to carry electrophotographic plate 9 under charging device 4 inpredetermined spacing from grille electrode 7. The return paths of beltsl0 and 11 are underneath guide plate 3. Plate 3 is supported on legssecured to the top of baseboard 1. The pulleys 19 are spaced apart tosupport electrophotographic plate 9 at two spaced regions across theplate. Insulating support blocks 6, formed of polystyrene or other goodinsulating material, comprise rectangular plates having their lowercorners beveled oil? at an angle of degrees. The electrode 7 is made ofconductive wire which is wound back and forth over support pins 21mounted on the lower and beveled edges of blocks 6 so as to form aparallel wire screen or cage partially enclosing corona discharge wires5, the electrode wires 7 being parallel to the corona wires. Preferably,the plane formed by a portion of the grille 7 and the plane formed bythe upper surface of each of the belts 10, 11 are parallel and atpredetermined relative positions with each other, which distances willbe later set out in more detail; but it is essential that the distancesbetween the corona discharge wires 5 and the electrode grille 7 is greatenough that no sparking will occurr at the voltages used to establishcorona.

An electrically conducting metal plate or shield 22 extends between thespaced insulating members 6 above the corona discharge wires 5. For bestoperating results this shield is held at a low potential, preferablyground, with respect to the discharge wires 5 and is approximately thesame distance from the discharge wires 5 as the grille 7.

5 in turn is connected to a source of DC. potential, as

a battery 32.

the a layer 2 is Mr".

".df bac'in meme. I ipr df te i i maimed against 1b w bla s,

The other end of secondary 30 is grounded.

Terminal 26 of control electrode 7 is connected by conductor 39 to a tap38 on the potentiometer 34 which If desired, the center tap of thepotentiometer 34 may be grounded so that the polarity of charge placedon the shield 7 may be varied as well as the magnitude of the potential.

A potential of at least about 4,000 volts between the corona electrodeand the nearest conductor or electrodes is usually required in order togenerate a useful corona discharge and it is preferable that the peakAC. voltage be no more than 10,000 volts in order to avoid sparking orexcessive space charges in structures of practical dimensions.

In the operation of the charging device of this invention, the relativespacing of the different elements has considerable effect on the resultsobtained. An example of spacings found to produce good results and toachieve the objects of this invention is such that the three coronadischarge wires 5 are spaced /2" fro-m each other in a single plane butbeing threaded through aligned mounting holes in the insulating members6. The shield 22 is spaced /2" above the wires 5, the wires of electrode7 in a plane parallel to the plane of the discharge wires 5 are in aplane @1 below the wires 5, and the spacing between this plane and theplane of travel of insulating The turns or wires of the electrode 7 haveproduced good results when they are spaced from each other byapproximately A suitable wire for corona wires 5 is a smooth stainlesssteel wire having a diameter of 0.0035 and a suitable wire for thestrands of electrode 7 is 0.01" stainless steel.

Under these conditions, 8,000 volts AC. was applied to the dischargewires 5 and a negative 800 volts D.C.

was applied to the electrode 7, the shield 20 being at ground potentialand a current was: found. to flow' from the wires- 5 to a metal platepassed under the charging devicein the position of plate 9.

It is, of course, obviousto those skilled. in theart' that the diameterof the corona wires 5 can be. made smaller or: larger provided the.voltage: applied to the wires. is suificient to generate. a. corona.discharge at a potential below that at. which sparking takes place.Likewise, the diameter. and spacing. of grille wires.7. can be variedover a rather wide: range although it is desirable. to use small wi'resso as to leave as. much open space as possible between. wires. whilekeeping the wire spacing small so. that the array of wires. 7causeanelectric field which resembles the field from a. plate electrodepositioned in the planes occupied, by the wires. It is also obviousthat.the charge ing. rate canhe increased. for any size of corona wires byincreasing voltage on the corona; wires, although it is essential that:the voltage. between these wires and electrodes 7 and 22 be below the.spark breakdown potential. The potential onwire electrode 7 can also bead'- justed to any desired valueto adjustthe potential which is appliedto the insulating layer 2.

i Fig. 4' is a graph illustrating certain advantages of the chargingdevice of this invention in preventing overcharging of the plate.Inobtaining this graph the charging device of Fig. 1' was modified'by.eliminating the shield 22 and. placing the control electrode 7 on theside of the corona 5 away from the plate 9. The control electrode 7 wasconstructed of sheet aluminum and had a rectangular cross sectionasshown in Fig. 5B. The. distance from the; corona wire 41 to each ofthe three sides of, the. control electrode 40 was A measured on aperpendicular line from the. corona to each surface. The distance fromthe corona wire. 4L to the surface of. the plate 9 was /2., A charge of,8,000'volts A.C. was applied to the discharge wires 41, and a currentwas. found to flow from. the wires 41 to a metal plate passed under thecharging device in the position of plate 9, the metal plate. beingconnected through a milliammeter to ground in order that the current,could be measured. Under. these conditions. the. DC. charge placed onelectrode 40 was varied as shown. in the graph. The amount of platecurrentwas. then determinedafor each setting of the potential on the.electrode 4.0.. This curve shows that the charging current to the platepasses through 0 for a particular determined. bias, i.e.. voltagebetween electrode 40 and the metal backing plate. When the plate ischarged to. a potential equivalent to. the point where place currentequals 0, no more current flows to the plate and, therefore, it isvimpossible. for the plate. to. reach: a: voltage higher than the voltageat this point in the curve. In contrast to this, wherea DC. potential isplaced on the corona electrode 41 and the D.C. bias placed on the shield40, the plot of the plate current against the. differ.- ence in voltagebetween the shield and the plate, instead of being a straight linepassing through 0 plate current, becomes asymptotic to the abscissa asplotted; that is, the plate current decreases markedly as a particularvoltage level isreached but, nevertheless, continues charging; abovethis voltage although at an' appreciably slower rate: The curveobtainedfor the control electrode-corona electrode configuration shownin Fig. 1 has the same shape as here, but will cross the 0 plate currentaxis at a different point-the exact point depending on the spacings ofthe electrodes and the plate. Thus, it is impossible to overcharge axerographic plate using the charging device of the instant inventionwhereas overcharging is possible when a DC. potential is placed on thecorona electrodes.

Figs. 2 and 3 illustrate difierent corona circuits. In Fig. 3 the switch33 places the DC. bias from a DC. source, as a battery 32 on coronashield 40 through lead 42. The metal backing portion 8 of thexerographic plate 9 is grounded by contact 35. This is a preferred formof the circuit as, in this case, no high D.C. charge is placed on. themetalpla-te. which: is handled by personnel: However,v as the. essentialfactor is: the charge relation; ship between. the metal backing: plate 8and. the shielfd; as shown. in; Fig:.2;. the: charge; if desired, maybeplaced by means; of switch 33 on the metal backing. member 8. while.grounding'the shield 40;. Resistor 31,desirab1yhas. ahighvalue, sayabout 10 megohms, to reduce the current. in. case. of, an accidentalgrounding of backing mem-. her 8.,

Fig. 5v illustrates in: cross section various, forms; of. operablecorona charging devices. As shown, the shape. of. the shield may take.awidely variant. number of terms; all of which are operable intheprocess of the instant invention. and all of which cause the platecharging 'currenalto pass. through. 0 forv a particular bias. In. anyevent, the control-v electrode. should be. parallel lengthwise. both tothe corona electrode and the insulating surfaceand should extendsufficiently far about the. corona electrode asto'preventcoronadischarge past the edges of the conductive electrode.When the control electrode is. positioned between the corona electrodeand the surface to be charged, as in Fig. l, the. control electrodeshould: be an apertured grid as parallel spaced conductive wires. asshown, wire mesh, etc. When the control electrode. is positioned on theside of. the corona electrode away from. the surface to be charged, asin. any of the forms. shown in Fig. SA-E, the control electrode may alsobe: anapertured grid or maybe a continuous conductive surface or acontinuous insulating surfacetwith parallel; spaced conductorsimbeddedtherein.

It is, obvious that, while thesurface to bev charged is. a.

plane inthe examples. illustrated, it. can take other forms,

such as. cylindrical, for. example, in which event the in,-sulatinglayer may be coated on.a rotating belt or drum.. It is alsoobvious that the insulating layer may beheld. stationary in the.apparatus during charging, and the. ion. source and control electrodecan. either be extensive. enough to cover. the entire surface or theycan be. ad.- vanced over the stationary insulatingv surface.

I claim:

1. A device for. appl'yingan electrostatic charge. to. an insulatinglayer comprising, in combination, means for supporting said. insulatinglayer in a charging plane, an: elongated corona discharge electrodeextending across. thewidth of said insulating layer. in spaced relationthereto,.means for producing relative travel. of said insulating layerand said corona discharge electrode in a direction substantiallyparallelto. the length of said. insulating layer, a control electrodeextending across the width of said insulating layer andparallellengthwise bothtothe coronav electrode and the insulating layerand extending sufficiently far about the corona electrode as to preventcorona. discharge past the edges of. the control electrode, said:control electrode being spaced from said corona discharge electrode andfrom saidinsulating layer, an. A.C. sourcev connected to said coronadischarge electrode for applying.

a corona generating A.C. voltage between said corona discharge electrodeand said control electrode to producean AC; corona discharge from saidcorona discharge electrode, and a DC; potential source connected tosaid. control electrode for applying a DC. voltage which is smaller thanthe peak value of said A.C. voltage to create a DC. charging fieldthrough the insulating layer whereby said control electrode willfunction to limit the potential which is applied to said insulatinglayer by said device.

2. A device for applying an electrostatic charge to an insulating layercomprising, in combination, means for supporting said insulating layerin a charging plane, an elongated corona discharge electrode extendingacross the width of said insulating layer in spaced relation thereto,means for producing relative travel of said insulating layer and saidcorona discharge electrode in a direction substantially parallel to thelength of said insulating layer, a control electrode extending acrossthe width of said insulating layer and parallel lengthwise both to thecorona electrode and the insulating layer and extending sumciently farabout the corona electrode as to prevent corona discharge past the edgesof the control electrode, said control electrode being spaced from saidcorona discharge electrode and from said insulating layer, an A.C.source connected to said corona discharge electrode for applying acorona generating A.C. voltage between said corona discharge electrodeand said control electrode to produce an A.C. corona discharge from saidcorona discharge electrode, and a D.C. potential source connected tosaid control electrode for applying a D.C. voltage which is smaller thanthe peak value of said A.C. voltage to create a D.C. charging fieldthrough the insulating layer, and means for varying the magnitude andpolarity of said D.C. voltage, whereby said control electrode willfunction to limit the potential which is applied to said insulatinglayer by said device.

3. A device for applying an electrostatic charge to an insulating layercomprising, in combination, means for supporting said insulating layerin a charging plane, an elonated corona discharge electrode extendingacross the width of said insulating layer in spaced relation thereto,means for producing relative'travel of said insulating layer and saidcorona discharge electrode in a direction substantially parallel to thelength of said insulating layer, and a control electrode comprising aconductive grille between said corona discharge electrode and saidinsulating layer across the width of said insulating layer, said controlelectrode being spaced from said corona discharge electrode and fromsaid insulating layer, an A.C. source connected for applying acorona-generating A.C. voltage between said corona discharge electrodeand said control electrode to produce an A.C. corona discharge from saidcorona discharge electrode, and a D.C. potential source connected tosaid control'electrode for applying a D.C. voltage which is smaller thanthe peak value of said A.C. voltage to create a D.C. charging fieldthrough the insulating layer, whereby said control electrode willfunction to limit the potential which is applied to said insulatinglayer by said device.

4. A device for applying an electrostatic charge to an insulating layercomprising, in combination, means for supporting said insulating layerin a charging plane, an elongated corona discharge electrode extendingacross the path of said insulating layer in spaced relation thereto,means for producing relative travel of said insulating layer and saidcorona discharge electrode in a direction substantially parallel. to thelength of said insulating layer, a control electrode extending acrossthe width of said insulating layer and partially surrounding said coronadischarge electrode on the side away from said insulating layer, saidcontrol electrode being spaced from said corona discharge electrode andfrom said insulating layer, an A.C.source connected to said coronadischarge electrode for applying a corona generating A.C. voltagebetween said corona discharge electrode and said control electrode toproduce an A.C. corona discharge from said corona discharge electrode,and a D.C. potential source connected to said control electrode forapplying a D.C.

1 8 voltage which is smaller than the peak value of said A.C. voltage tocreate a D.C. charging field through the insulating layer, whereby. saidcontrol electrode will function to limit the potential which is appliedto said insulating layer by said device.

5. A device for applying an electrostatic charge to an insulating layercomprising, in combination, means for supporting saidinsulating layer ina charging plane, an elongated corona discharge electrode extendingacross the width of said insulating layer in spaced relation thereto,means for producing relative travel of said insulating layer and saidcorona dischargeelectrode in a direction substantially parallel to thelength of said insulating layer, a control electrode extending acrossthe width of said insulating layer and partially surrounding said coronadischarge electrode on the side away from said insulating layer, saidcontrol electrode being spaced from said corona discharge electrode andfrom said in sulating layer, an A.C. source connected to said coronadischarge electrode for applying a corona generating A.C. voltagebetween said corona discharge electrode and said control electrode toproduce an A.C. corona discharge from said corona discharge electrode,and a D.C. potential source connected to said control electron forapplying a D.C. voltage which is smaller than the peak value of saidA.C. voltage to create a D.C. charging field through the insulatinglayer, and means for varying the magnitude and polarity of said D.C.voltage, whereby said control electrode will function to limit thepotential which is applied to said insulating layer by said device.

6. A device according to claim 5 wherein the control electrode issemi-cylindrical.

7. A device according to claim 5 wherein the control 7 electrode has arectangular cross section.

8. A device according to claim 5 wherein the control electrode is ofuniformly-spaced parallel wires.

9. A device according to claim 5 wherein the control electrode has acontinuous conductive surface.

10. A method of sensitizing a photoconductive insulating layer byapplying electric charge to its surface comprising applying anA.C.corona generating potential to a corona discharge electrodeextending across the width of the layer to be charged and extendingsubstantially parallel thereto while spaced apart therefrom, creating aD.C. charging field through the photoconductive insulating layer, andmoving said-electrode relative to said photoconductive insulating layerand in a direction substantially parallel to the length of the surface.

References Cited in the file of this patent UNITED STATES PATENTS2,132,707 Smith Oct. 11, 1938 2,143,214 Selenyi Jan. 10, 1939 2,543,051Oughton et al Feb. 27, 1951 2,647,464 Ebert Aug. 4, 1953 2,693,416Butterfield Nov. 2, 1954 2,701,764 Carlson Feb. 8, 1955 2,741,959Rheinfrank Apr. 17, 1956 r 2,777,957 Walkup Jan. 15, 1957 UNITED STATESPATENT OFFICE CERTIFICATE OF CORRECTION Patent N0. 2, 879,395 March 24,1959 Lewis E. Walkup- It is hereby certified that error appears inthe-printed specification of the above numbered patent requiringcorrection and that the said Letters Patent should read as correctedbelow.

line 65, for "in", second occurrence, read to column ,000 volts" read4,000 and 10,000 volts column 4., line 23, for "plate" read platescolumn '7, line 22, for "e'lonated" read elongated line 32, after"connected" insert to said corona discharge electrode line 47, for "pathread Width column .8, line 25, for "electronn read electrode Column 1,2, line 28, for "4,000 to 10 Signed and sealed this 22nd day'of" March1960.

(SEAL) Attest:

KARL H. AXLINE Attesting Officer ROBERT C. WATSON Commissioner ofPatents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.2,879,395 March 24, 1959 Lewis E Walkup It is herebj certified thaterror appears in the-printed specification of the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 1, line 65, for "in", second occurrence, read to column 2, line28, for "4,000 to 10,000 volts" read 4,000 and 10,000 volts column 4,line 23, for "plate" read plates 3 column '7, line 22, for "e-lonated"read elongated line 32, after "connected" insert to said coronadischarge electrode line 47, for "path" read Width column 8, line 25,for "electron" read electrode Signed and sealed this 22nd day of March1960.

(SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON Attesting Ofiicer Commissioner ofPatents

